Next.js Best Practices Skill

Apply these rules when writing or reviewing Next.js code. Covers App Router (Next.js 13+).

1. File Conventions

Project Structure

app/
├── layout.tsx           # Root layout (required)
├── page.tsx             # Home page
├── loading.tsx          # Loading UI (Suspense boundary)
├── error.tsx            # Error boundary
├── not-found.tsx        # 404 page
├── global-error.tsx     # Root-level error boundary
├── (auth)/              # Route group (no URL segment)
│   ├── login/page.tsx
│   └── register/page.tsx
├── [id]/                # Dynamic segment
│   └── page.tsx
├── [...slug]/           # Catch-all segment
└── api/
    └── route.ts         # Route Handler

Special Files

2. React Server Components (RSC) Boundaries

Server Components (default) - do:

• async/await data fetching directly in component
• Access databases, file system, environment variables
• Import server-only packages

Client Components ('use client') - required when:

• Using useState, useEffect, useReducer, useContext
• Using browser APIs (window, document, localStorage)
• Using event handlers (onClick, onChange)
• Using third-party client libraries

Rules

• Never make a Client Component async - it's invalid
• Never pass non-serializable props (functions, class instances) from Server → Client
• Server Components can import Client Components, not the reverse
• Use 'use server' only for Server Actions, not to mark Server Components

3. Async APIs - Next.js 15+

In Next.js 15, params, searchParams, cookies(), headers(), and draftMode() are now async. Always await them:

// ✅ Correct - Next.js 15+
export default async function Page({
  params,
  searchParams,
}: {
  params: Promise<{ id: string }>
  searchParams: Promise<{ q: string }>
}) {
  const { id } = await params
  const { q } = await searchParams
  const cookieStore = await cookies()
  const token = cookieStore.get('token')
  // ...
}

// ❌ Wrong - old synchronous pattern
export default function Page({ params }: { params: { id: string } }) {
  const { id } = params  // broken in Next.js 15
}

4. Data Fetching Patterns

Server Components - preferred for most fetching

async function ProductList() {
  const products = await db.products.findMany() // Direct DB access, no API needed
  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>
}

Avoid waterfalls - use Promise.all

// ❌ Sequential waterfall - slow
const user = await getUser(id)
const posts = await getPosts(user.id)

// ✅ Parallel - fast
const [user, posts] = await Promise.all([getUser(id), getPosts(id)])

Server Actions - for mutations

// app/actions.ts
'use server'

export async function createPost(formData: FormData) {
  const title = formData.get('title') as string
  await db.posts.create({ data: { title } })
  revalidatePath('/posts')
}

// app/page.tsx
import { createPost } from './actions'

export default function Page() {
  return (
    <form action={createPost}>
      <input name="title" />
      <button type="submit">Create</button>
    </form>
  )
}

Route Handlers - for API endpoints needed by external clients

// app/api/products/route.ts
import { NextRequest, NextResponse } from 'next/server'

export async function GET(request: NextRequest) {
  const products = await db.products.findMany()
  return NextResponse.json(products)
}

export async function POST(request: NextRequest) {
  const body = await request.json()
  const product = await db.products.create({ data: body })
  return NextResponse.json(product, { status: 201 })
}

5. Image Optimization

Always use next/image instead of <img>:

import Image from 'next/image'

// Local image - size known automatically
import photo from './photo.jpg'
<Image src={photo} alt="Description" />

// Remote image - must configure domain in next.config.ts
<Image
  src="https://example.com/photo.jpg"
  alt="Description"
  width={800}
  height={600}
  sizes="(max-width: 768px) 100vw, 50vw"
  priority  // Add for LCP images above the fold
/>

Configure remote domains:

// next.config.ts
const config: NextConfig = {
  images: {
    remotePatterns: [
      { protocol: 'https', hostname: 'example.com' },
    ],
  },
}

6. Font Optimization

Always use next/font instead of <link> tags - it prevents layout shift and self-hosts fonts:

// app/layout.tsx
import { Inter, Roboto_Mono } from 'next/font/google'

const inter = Inter({
  subsets: ['latin'],
  display: 'swap',
  variable: '--font-inter',
})

export default function RootLayout({ children }) {
  return (
    <html className={`${inter.variable}`}>
      <body>{children}</body>
    </html>
  )
}

7. Error Handling

// app/error.tsx - must be a Client Component
'use client'

export default function Error({
  error,
  reset,
}: {
  error: Error & { digest?: string }
  reset: () => void
}) {
  return (
    <div>
      <h2>Something went wrong!</h2>
      <button onClick={reset}>Try again</button>
    </div>
  )
}

Trigger programmatically:

import { notFound, redirect } from 'next/navigation'

// In any Server Component or Server Action:
if (!user) notFound()                          // renders not-found.tsx
if (!session) redirect('/login')               // HTTP 307 redirect

8. Metadata

// app/layout.tsx - static
export const metadata = {
  title: { template: '%s | My App', default: 'My App' },
  description: 'My application description',
}

// app/products/[id]/page.tsx - dynamic
export async function generateMetadata({ params }) {
  const { id } = await params
  const product = await getProduct(id)
  return {
    title: product.name,
    openGraph: { images: [product.imageUrl] },
  }
}

9. Suspense Boundaries

Wrap anything that uses useSearchParams() or usePathname() in a Client Component with <Suspense> to avoid CSR bailout:

import { Suspense } from 'react'
import SearchBar from './SearchBar'  // contains useSearchParams

export default function Page() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <SearchBar />
    </Suspense>
  )
}

10. Self-Hosting with Docker

// next.config.ts
const config: NextConfig = {
  output: 'standalone',
}

FROM node:20-alpine AS base
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:20-alpine AS runner
WORKDIR /app
COPY --from=base /app/.next/standalone ./
COPY --from=base /app/.next/static ./.next/static
COPY --from=base /app/public ./public
CMD ["node", "server.js"]

Common Mistakes

• Async Client Components - invalid, move async to a parent Server Component
• params access without await in Next.js 15+ - add await params
• <img> instead of next/image - always use next/image for optimization
• <link> tags for fonts - always use next/font
• Fetching in Client Components when a Server Component would work - default to Server
• Route Handler + page.tsx in the same folder - causes a conflict, use different paths
• export const dynamic = 'force-dynamic' everywhere - use it only when truly needed